given an iterator, declare an index with correct size_type

Question

In a C++ template, I struggle to define a variable with the correct size_type. Basically, this will be an index type into the container. I know that int works but would like to have it in a clean form.

template<typename ForwardIt> 
void test(ForwardIt it) {
    // this yields the underlying type
    decltype(typename std::iterator_traits<ForwardIt>::value_type()) T;

    // what I would like is something like above, but yielding the
    // correct size_type for the container.

    // Something like this but with the type extracted from ForwardIt:
    std::vector<int>::size_type i; 

    ...
}

Answer 1

Elaborating on what @NathanOliver said: Iterators don't have size_types; they have a difference_type, which represents the distance between two iterators. Containers have a size_type.

An iterator need not have an associated container, so there's no way to get "the container's size_type" from just an iterator.